You know, now when you mention it ...
There is only single narative reason to step inside that tomb ... and that is fact that those looters, or explorer, or graverobbers, or whatever you wish to call them ... are searching there for something valuable ... that, or general curiocity.
But we agreed on that being enough.
Now if you concider Swamp:
Either Ethel will invite you there, if you talk to her in Tiefling part of Groove ...
Or you meet her again arguing with those two peasants as you explore around Blighted Village ...
Or you went there to find out what is Kagha up to ...
Or you can appear there by pure coincidence, as you were exploring Underdark searching for the way to Moonrise tower ...
Or you can get there by even purer accident, when you try to sneak in Goblin camp around that casm, and rotten log will crack as you get over it. 
If you take Risen Road:
Lae'zel is urging you to get there to find her kin ...
Or, you find out burning inn from Grove, if you use that Telescope ...
Or, im not quite sure about this one, but i would swear that nobody give you exactly best description about where to find Goblin camp, so i would dare to say that is quite possible to end up there while you are searching for it ... 
Also, that road leads straight towards Baldur's Gate ... so if that is where you go, it would be quite logical for you to take this way ...